If or when all else has failed why not try the (my?) "two identical antenna
method"? Set up a pair of two identical short whips about 30 to 50m apart
on reasonably uniform ground. they should be identically loaded /matched
preferably to 50 ohms. Do not worry about how much loss you have for each
of these. It factors out. Measure x (dB)= the path loss in dB
between them, with Tx on one and Rx on the other. You should use a source
of sufficient power to give a good SNR at the receive end. Then place your
vehicle with whip at the half wave point. Put a known power Pin (dBW) into
the Tx whip. The power density or erp at the halfway point is then
Pin(dBW) - x/2(dB). Measure the signal strength on the car whip. You may
then convert this into volts per metre if you wish. This method should
work for any of the LF bands. In theory it should also work at
9kHz!?
Hope this turns out to be useful.
Sent: Thursday, April 01, 2010 2:05
PM
Subject: Re: LF: Re: E-Field Probe
calibration question
Hi Alan + All...
Actually its a bit more complicated on its own... The
physical length of the element is 0.75m, which is appreciably longer than the
electrical effective length. And I'm betting a chunk of that
difference is made up by having a series resistor in the output circuitry to
give a 50 ohm source impedance and help stability but reducing voltage gain to
below unity. If 50R in series, there is an immediate -6dB halving
the length. If my original dipole concept were to
apply, then the other half of the leg caused by the car would also be affected
by the attenuation.
But it appears from your comments so far the argument doesn't apply in
which case the 0.15m length still holds. How do the EMC
measurement community do it, as their measurements have to be valid in law and
could be held up to scrutiny?
Now, what was the voltage from an untuned loop, again ;-?
Andy
Mostly retired inguneer
On 1 April 2010 12:44, Alan Melia
<[email protected]>
wrote:
Hi Andy maybe I dont read it right, but if that is an
E-field probe it is
not a "dipole" The input capacitance senses the
voltage at two points on the
wavefront. The ground plane is, I believe,
irrelevant. You just need another
capacitance separated from the "probe"
to reference to voltage to. See
Renato's balanced probe (www.vlf.it) In your case
this is the car.....but
it could be the size of a dinky toy.The area of
the wavefront intercepted by
the probe or the ground reference does not
affect the terminal
voltage.....as I see it.................of course I
might just be wrong cos
i am really a scientist not an
enjuneir!!
:-))
Alan
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